Frequency referencing system and method

ABSTRACT

A system and method of referencing frequencies of radiation from a plurality of sources, for example lasers (1,2) includes filter element (11), such as a Fabry-Perot etalon, onto which the radiation is incident. The filter element has a replicated set of passbands spaced apart in frequency. Detection means (12, 13) detects radiation passing through the filter element (11) and provides corresponding output signals. Control means (16-18, 3, 4) is responsive to the output signals from the detection means (12, 13) to control the sources (1, 2) so that the radiation transmitted through the filter element (11) is maintained substantially constant. The bandwidths of the radiation are small compared with the corresponding passbands of the filter element. The invention is particularly applicable in a communications network in which a number of sets of remote sources of radiation are referenced to a central set of reference sources.

There is an increasing requirement particularly in optical widebandnetworks closely to control the wavelength or frequency of thetransmitted signals. Indeed, the more channels which are transmittedaround the network the more important it is to control the frequency ofthe signals.

In accordance with one aspect of the present invention, a method ofreferencing the frequencies of radiation from a plurality of sourcescomprises guiding the radiation to a filter element having a replicatedset of passbands spaced apart in frequency; monitoring radiationtransmitted through the filter element; and controlling the sources sothat the radiation transmitted through the filter element is maintainedsubstantially constant, the bandwidths of the radiation being smallerthan the corresponding passbands of the filter element.

In accordance with a second aspect of the present invention, a frequencyreferencing system comprises a plurality of sources of radiation ofdifferent frequencies; a filter element on to which the radiation isincident, the element having a replicated set of passbands spaced apartin frequency; detection means for detecting radiation passing throughthe filter element and for providing corresponding output signals; andcontrol means responsive to the output signals from the detection meansto control the sources so that the radiation transmitted through thefilter element is maintained substantially constant, the bandwidths ofthe radiation being smaller than the corresponding passbands of thefilter element.

The invention provides a particularly simple way to lock the frequenciesof the radiation from the sources relatively to one another. It is thusparticularly suitable, when the radiation is optical radiation, inoptical wideband networks as described above.

The invention is also applicable to radiation at non-optical frequenciessuch as radio and microwaves.

The monitoring step may comprise monitoring the intensity of thetransmitted radiation. In this case the bandwidths of the radiation arepreferably offset from the central frequency of the correspondingpassbands of the filter element and particularly conveniently they arepositioned close to the edge of the corresponding passbands. Theadvantage of this is that any change in relative position between theradiation frequency and the filter element passband will correspond withan increase or decrease in the intensity of the radiation transmittedthrough the filter element depending on the direction of the change.

Alternatively, the phase of the transmitted radiation may be monitored.In this case the radiation bandwidth may be substantially centered onthe corresponding passband and any change in the relative positions ofthe radiation frequency and the passband will result in a phase changein the transmitted beam the type of phase change differing with thedirection of movement between the radiation frequency and passband. Theadvantages of this are that there is no attenuation since the beam iscentered on a passband, and any dc drifts will not affect performancesince phase and not intensity is monitored.

In some cases, the passbands of the filter element will shift togetherin response for example to temperature changes. Preferably, thereforeone source comprises a reference source for generating a reference beam,the method further comprising monitoring transmitted radiationcorresponding to the reference beam, and adjusting the filter element tomaintain the transmission characteristic of the reference beamsubstantially constant. For example, the system may include means foradjusting the filter element in response to output signals from thedetection means corresponding to radiation from the reference source.

The adjusting means may comprise means for physically moving orstressing the filter element such as a piezo transducer or a steppermotor or means for adjusting the temperature of the filter element orits support such as a Peltier cooler.

Preferably, the passbands of the filter element are substantiallyequally spaced apart in frequency.

The filter element may be provided in the case of optical radiation byan optical waveguide loop or ring resonator which passes radiationhaving the fundamental frequency of the loop and also higher resonantfrequencies but is most conveniently provided by a Fabry-Perot etalon.An etalon has a power transfer characteristic, usually at opticalfrequencies, which exhibits passbands at regular frequency intervalsover a wide frequency range. The width of the passbands and theirspacing are determined by the mechanical dimensions of the etalon and bythe materials and coatings. Etalons may be made for example by usingsilica glass or air spaced devices. Integrated optic or fiber baseddevices may be used to achieve a similar effect. The spacing betweenpassbands may range from for example 10's of MHz to several nm.

In one example, the radiation from each source may be incident ondifferent parts of the filter element. This enables the differentradiation beams to be easily distinguished but requires the use of acorresponding plurality of detectors.

Preferably, therefore, radiation from the plurality of sources is guidedalong the same path to the filter element, common detection means beingprovided to receive the radiation transmitted through the filterelement, and the control means being arranged to impart respectiveidentifiers to the radiation from each source and to detect from theradiation passing through the filter element each identifer and theradiation corresponding to the identifier.

The identifier may comprise a frequency or time code modulation.

Typically, the sources will comprise lasers.

In some cases, it may be desirable to provide several sets of sourceswhose frequencies are referenced to each other, the sets of sourcesbeing widely spaced apart physically. The frequencies of these sets ofsources could be referenced by providing a reference source whose outputis fed to each set of reference sources each of which is provided withits own filter element. However, this requires that each filter elementis perfectly matched and in an optical communications network anymismatch could adversely affect switching or transmission performance.

Conveniently, a communications network comprises a frequency referencesystem in accordance with the second aspect of the invention in whicheach source of radiation comprises a reference source, multiplexingmeans for multiplexing the frequency referenced radiation from thereference sources and splitting means for splitting the multiplexedradiation into a plurality of subsidiary signals each of which is fed toa respective set of remote sources of radiation; and subsidiary controlmeans at each set of subsidiary sources for referencing the frequenciesof the radiation from the subsidiary sources to one or more of thereference frequencies supplied from the splitting means.

Some examples of systems and methods in accordance with the presentinvention will now be described with reference to the accompanyingdrawings which are schematic block diagrams, and in which:

FIG. 1 illustrates one example of the invention;

FIG. 2 illustrates the transmission characteristics of the filterelement shown in FIG. 1;

FIG. 3 illustrates a modification of the example shown in FIG. 1;

FIG. 4 illustrates a second example;

FIG. 5 illustrates a third example; and,

FIGS. 6 and 7 illustrate a fourth example.

FIG. 1 illustrates two lasers 1,2 driven by respective laser drivers 3,4 which generate optical output signals having bandwidths centered ontwo different frequencies. The optical signals are guided along opticalwaveguides 5, 6, such as optical fibers, to respective fiber splitters7,8. A portion of the signal fed to each splitter 7, 8 is diverted torespective converging lenses 9, 10. Light from the lenses 9, 10 isfocused onto a Fabry-Perot etalon 11 at spaced apart locations. Opticalradiation which passes through the etalon 11 is received by respectiveconverging lenses 12, 13 and guided along optical fibers 14, 15 torespective photodiodes 16, 17.

The photodiodes 16, 17 provide electrical outputs corresponding to theintensity of the light passing along the optical fibers 14, 15, theelectrical outputs being fed to comparators 18, 19.

The transmission characteristic of the etalon 11 is illustrated in FIG.2. This indicates that the etalon 11 has a plurality of passbands 20substantially equally spaced in frequency. The spacing between thepassbands 20 may range from 10's of MHz to several nm.

If the output signal from one of the lasers 1,2 has a frequency fallingwithin one of the passbands 20 it will be transmitted through the etalon11, the intensity of the transmitted signal depending upon the frequencyof the signal relative to the passband. In fact, it is preferable if thefrequency of the optical signal is not exactly centered within thepassband 20 so that any change in the intensity of the transmittedsignal can be correlated easily with the direction of relative movementof the passband and the optical signal frequency.

In an alternative arrangement (not shown) either the laser output isfrequency modulated or the etalon frequency spacing is modulated and thephotodiodes 16, 17 are followed by phase detectors. In either case, thefrequency of the optical signals may be centered on correspondingpassbands and in either case the interaction of the radiation andoptical filter causes intensity changes in the received signal. Theamplitude of the detected sub-carrier indicates the instantaneousdifference between laser frequency and etalon passband and the phaseindicates the relative direction of movement.

The control of the laser 1 will now be described in detail and it shouldbe understood that similar steps would be carried out to control thelaser 2. Initially, a desired output frequency for the optical signalfrom the laser 1 is set. This output frequency must fall within one ofthe passbands of the etalon 11 so that in use a signal will be receivedby the photodetector 17. When the frequency of the optical signal forthe laser 1 is at the desired magnitude a reference input to thecomparator 19 is set so that the laser driver 3 controls the laser 1 tooutput an optical signal of the required frequency. If there is a changein the frequency of the signal from the laser 1 due for example tochanges in temperature the signal passing into the waveguide 15 willhave a different intensity from its desired value. An electrical signalcorresponding to the intensity of the incoming optical signal isgenerated by the photodiode 17 and is fed to the comparator 19. Sincethe comparator 19 will detect a difference between the signal from thephotodiode 17 and the reference signal it will cause an appropriateelectrical signal to be fed to the laser driver 3 to adjust thefrequency of the optical signal generated by the laser 1. In this way,the output signal from the laser 1 is maintained at a substantiallyconstant frequency.

The laser 2 is similarly controlled but at a frequency corresponding toa different passband of the etalon 11. Since the spacing in frequencybetween the passbands of the etalon 11 is substantially fixed, theoutput signals from the lasers 1, 2 will also remain fixed relatively toone another despite changes in temperture or other effects.

For a given etalon, the precise spacings between passbands may vary withtemperature. If in FIG. 1, temperature changes cause the etalonfrequency to shift then, because of the control loops, both lasers 1, 2will follow the shift and preserve their relative frequency. However, insome applications an absolute stability may be needed which is beyondthe stability limits of the etalon. Improved stability may be obtainedby means of a control loop in which the angle of the etalon 11 iscontrolled with respect to the optical axis. This is illustrated in FIG.3. Tilting the etalon 11 in this way causes a slight change in thedevice optical path length and hence a change in resonant frequency andwill thus restore the passbands of the etalon to their originalpositions or frequencies.

In this modification, the etalon 11 itself is locked onto a more stablereference such as a HeNe laser 21 or to a laser locked to an atomicreference. As in the FIG. 1 example, the optical output signal from thelaser 21 is guided to the etalon 11 and the transmitted signal isreceived by a photodiode 22. This is fed to a comparator 23 whose outputis fed to a positional control unit 24. The positional control unitcould be either a piezo transducer of a stepper motor.

A particularly attractive method of stabilizing the etalon 11 is toplace a crystal of LiNbO₃ in between the partially reflecting mirrors ofthe etalon. By controlling the electric field across the LiNbO₃ theoptical pathlength may be varied. This approach would remove the needfor mechanical movement of the etalon 11.

The FIG. 1 example and indeed its modification require that each opticalsignal from the lasers is fed to a different part of the etalon 11. Thisrequires that separate photodiodes are provided for each laser. Analternative example which simplifies the construction of the etalon andreduces the number of photodiodes required is illustrated in FIG. 4. Inthis example, a plurality of lasers are provided (only two of which areshown in the drawing) whose output signals are fed to fiber splitters sothat a portion of the output signals are guided to an optical combiner25. The output of the optical combiner 25 is fed to a single converginglens 26, through the etalon 11 to a single lens 27 and from the lens 27to a common photodiode 28. The electrical output from the photodiode 28is fed to an electrical demultiplexer 29 having one output for each ofthe lasers.

To distinguish between the optical signals from each laser, a modulator30 is associated with each laser, the modulators 30 imparting a uniqueidentification modulation to the output signals from the lasers. Themodulation could be, for example either by frequency or time. Iffrequency is used then a unique frequency code is allocated to eachlaser, if time, then a unique time slot or time code is allocated. Theelectrical demultiplexer 29 senses the identification modulation andprovides electrical output signals relating to the intensity of theradiation corresponding to each identification modulation.

FIG. 5 illustrates a system for locking a laser to a reference laserbased on the FIG. 1 example and the modification of FIG. 3. A referencelaser 31 such as a HeNe laser provides a reference optical signal whichis fed to an etalon 32. The output signal from a laser 33 to bereferenced is split so that part of the output signal is also fed to theetalon 32. Optical signals transmitted through the etalon 32 are splitwith the transmitted reference signal being fed to a photodiode 34 andto a photodiode 35 while the transmitted signal from the laser 33 is fedto a photodiode 36 and to the photodiode 35. The output signal from thephotodiode 34 is fed via a buffer 37 and a control interface 38 to apositional control unit 39 similar to that shown in FIG. 3. The outputfrom the photodiode 36 is fed via a buffer 40 whose output is split withone portion being fed to a counter 41 and the other portion to a threeposition switch 42. The output of the photodiode 45 is fed to a filter43, a comparator 44, and an electrical detector 45 whose output is alsofed to the switch 42. The laser 33 is driven by a laser driver 46.

Operation of this system is as follows:

1. The position of the etalon 32 is locked so that one of its passbandscoincides with the reference frequency from the laser 31. This isachieved using the control loop incorporating the photodiode 34 and thepositional control unit 39.

2. With the switch 42 in position (a), the frequency of the laser 33 isadjusted by adjusting a control A which feeds one input of thecomparator 44 so that the laser 33 produces a beat with the referencesignal to produce an electrical IF signal at B. The frequency of thesemiconductor laser 33 is now known.

3. The switch 42 is then moved to the position (b) and the semiconductorlaser frequency is adjusted to the required wavelength by applying asuitable offset to the laser driver 46. Each time the laser frequencypasses through a passband of the etalon 32, a high level signal appearsat C. The counter 41 records the number of times this occurs so that,knowing the frequency of the laser 31 and the passband spacing at theetalon 32, the frequency of the laser 33 is also known.

4. When the required frequency is reached, the switch 42 is set to theposition (c) and the laser control loop stabilizes the frequency to therequired etalon passband as previously described.

In the preceding examples, the reference laser is shown co-sited withthe semiconductor lasers being locked. These schemes would also allowthe reference laser to be sited remotely from the locked lasers. Severalsets of locked lasers might be sited at different locations remote fromthe reference laser in which case each set could have its own feederfrom the reference laser and its own F-P etalon. In these circumstances,unless the etalons were perfectly matched, small differences infrequency would appear between the locations. In an opticalcommunications network this could adversely affect switching ortransmission performance. FIGS. 6 and 7 illustrate an arrangement todeal with this problem.

The section in dashed lines in FIG. 6 corresponds to the FIG. 7 examplewith a majority of the locking system illustrated schematically at 47.The locked signals from the lasers two of which are indicated at 1, 2,with their identification modulations are fed to an opticalcombiner/splitter 48 so that the optical signals are combined to form anoptical frequency multiplex and then transmitted to a number of remotelocations. At each location there is provided an optical splitter 49having one output for each laser of the set of lasers at that location.

Each laser of the set of lasers is connected in an opto-electronicfrequency control loop of the form illustrated in FIG. 7. Themultiplexed reference signals are fed from the splitter 49 along anoptical fiber 50 to a photodiode 51, an electrical IF processor 52 andan electrical detector 53. The output from the detector 53 is fed to abuffer 54 whose output is fed to a laser driver 55 which controls thelaser 56. The output signal from the laser 56 is split at 57 with aportion of the output signal being fed back to the photodiode 51.

Initially, the loop is locked to any one of the reference signals in theincoming multiplexed signal via the path just described. An electricalbeat frequency will appear at the output of the photodiode 51 and thisis detected. The detected signal will contain the identificationmodulation corresponding to the reference frequency with which the laser56 is locked and this identifier is fed to a code check circuit 58. Thisdetermines in conjunction with a control unit 59 if and how the locallaser 56 should be retuned. An appropriate correction signal is thenadded to the control signal in the buffer 54 to retune the laser 56 to arequired channel. When this is completed, control continues via thedirect loop path.

We claim:
 1. A method of referencing the frequencies of radiation from aplurality of sources, the method comprising:guiding the radiation fromeach source to a filter element having a replicated set of passbandsspaced apart in frequency; monitoring radiation at each of thewavelengths transmitted through the filter element; and controlling thesources so that the radiation at each wavelength transmitted through thefilter element is maintained substantially constant, the bandwidths ofthe radiation at each wavelength being smaller than the correspondingpassbands of the filter element.
 2. A method according to claim 1,wherein the intensity of the transmitted radiation is monitored, thebandwidths of the radiation being offset from the central frequency ofthe corresponding passbands of the filter element.
 3. A method accordingto claim 1, wherein either the radiation frequency is modulated or thefrequency spacing of the passbands of the filter element is modulated,the amplitude and phase of the detected sub-carrier being monitored. 4.A method according to anyone of claims 1 to 3, wherein one sourcecomprises a reference source for generating a reference beam, the methodfurther comprising monitoring transmitted radiation corresponding to thereference beam, and adjusting the filter element to maintain thetransmission characteristic of the reference beam substantiallyconstant.
 5. A method according to anyone of the preceding claims, 1, 2or 3 further comprising imparting respective identifiers to theradiation from each source; guiding the radiation from each source alonga common path to the filter element; detecting from the radiationtransmitted through the filter element each identifier; and detectingthe radiation corresponding to the identifiers.
 6. A method according toclaim 5, wherein the identifier comprises a frequency or time codemodulation.
 7. A method according to anyone of the preceding claims 1, 2or 3 wherein the radiation is optical radiation.
 8. A frequencyreferencing system comprising:a plurality of sources of radiation ofdifferent frequencies; a filter element having a replicated set ofpassbands spaced apart in frequency on to which the radiation from eachsource is incident; detection means for detecting the intensity of theradiation at each wavelength passing through the filter element and forproviding output signals corresponding to each wavelength; and controlmeans responsive to the output signals from the detection means tocontrol the sources so that the radiation at each wavelength transmittedthrough the filter element is maintained substantially constant, thebandwidths of the radiation at each wavelength being smaller than thecorresponding passbands of the filter element.
 9. A system according toclaim 8, wherein one source comprises a reference source the systemfurther including means for adjusting the filter element in response tooutput signals from the detection means corresponding to radiation fromthe reference source.
 10. A system according to claim 8 or claim 9,wherein the passbands of the filter element are substantially equallyspaced apart in frequency.
 11. A system according to any one of claims 8or 9, wherein the filter element comprises a Fabry-Perot etalon or aring resonator.
 12. A system according to anyone of claims 8 or 9,wherein radiation from the plurality of sources is guided along the samepath to the filter element, common detection. means being provided toreceive the radiation transmitted through the filter element, and thecontrol means being arranged to impart respective identifiers to theradiation from each source and to detect from the radiation passingthrough the filter element each identifier and the radiationcorresponding to the identifier.
 13. A system according to anyone ofclaims 8 or 9, wherein the sources comprise lasers.
 14. A communicationsnetwork comprising a frequency referencing system according to anyone ofclaims 8 or to 9 in which each source of radiation comprises a referencesource, multiplexing means for multiplexing the frequency referencedradiation from the reference sources and splitting means for splittingthe multiplexed radiation into a plurality of subsidiary signals each ofwhich is fed to a respective set of remote sources of radiation; andsubsidiary control means at each set of subsidiary sources forreferencing the frequencies of the radiation from the subsidiary sourcesto one or more of the reference frequencies supplied from the splittingmeans.
 15. A method for simultaneously controlling the operating opticalfrequencies of plural lasers, said method comprising the stepsof:passing the optical radiation output from each of said plural lasersthrough a single common optical frequency filter having a plurality ofpassbands spaced apart in the optical frequency domain, the output ofeach laser being controllable in frequency to align with a respectiveone of said passbands in the frequency domain; processing the opticalradiation passed through said filter to derive a plurality of frequencycontrol electrical signals, each of said control electrical signalscorresponding to the radiation passed through a respectivelycorresponding passband of the filter by a respectively correspondinglaser; and using said plural frequency control electrical signals toactively control the operating optical frequency of its respective laserand maintain such operating frequency within its associated filterpassband.
 16. A method as in claim 15 further comprising the step ofalsopassing reference optical radiation from a reference laser ofpredetermined stable operating frequency through said single commonoptical frequency filter; and using optical radiation from the referencesource passed by the filter to actively control the frequency of theplural filter passbands.
 17. A system for simultaneously controlling theoperating optical frequencies of plural lasers, said systemcomprising:plural controllable frequency laser sources of opticalradiation; a common optical frequency filter disposed to interceptradiation emanating from each of said laser sources, said filter havinga plurality of passbands spaced apart in the optical frequency domain;feedback control circuits for converting radiation passed by said filterinto a plurality of electrical control signals, each beingrepresentative of the alignment of a respectively associated filterpassband and laser output frequency and for applying said electricalsignals to control the respective laser output frequencies and thusmaintain each with an associated passband of the filter.
 18. A system asin claim 17 further comprising:a reference laser having a stablepredetermined operating optical frequency and arranged to direct atleast some of its optical output through said filter; and filterfrequency control means connected and disposed to receive referencefrequency radiation from the reference laser and passed by the filterand to control the filter to maintain one of its passbands alignedtherewith in the frequency domain.